In recent years, R-T-B-based alloys, which exhibit good magnetic properties, have been available as a rare earth magnet alloy. In the term “R-T-B-based alloys” as used herein, “R” refers to rare earth metals, “T” refers to transition metals with Fe being an essential element, and “B” refers to boron. Alloy flakes of R-T-B-based alloys can be produced by a rapid solidification process. In a rapid solidification process, a molten R-T-B-based alloy is prepared by heating raw materials, and the molten alloy is supplied to a chill roll and solidified thereon so that it can be cast into a ribbon. For the rapid solidification process, strip casting methods are widely used.
When a strip casting method is employed for the rapid solidification process, rare earth alloy flakes may be produced by the following procedure, for example:
(a) A molten R-T-B-based alloy is prepared by loading raw materials into a crucible and heating and melting them;
(b) The molten alloy is supplied, via a tundish, to the outer peripheral surface of a chill roll having a structure in which coolant circulates. With this, the molten alloy is quenched and solidified to be cast into a ribbon having a thickness of 0.1 to 1.0 mm;
(c) The cast ribbon is crushed into alloy flakes, and the alloy flakes are cooled.
In this procedure, the above operations (a) to (c) are usually carried out under reduced pressure or in an inert gas atmosphere to prevent oxidation of the R-T-B-based alloy.
Such casting of ribbons by a rapid solidification process using a chill roll is typically carried out in a batch manner because of the limited capacity of a crucible for melting the loaded raw materials into the molten alloy. Furthermore, the chill roll is repeatedly used over several casting operations.
Rare earth alloy flakes produced by a rapid solidification process have an alloy crystal structure in which a crystalline phase (principal phase) and an R-rich phase coexist. The crystalline phase is an R2T14B phase, and the R-rich phase is enriched with the rare earth metal. The principal phase is a ferromagnetic phase that contributes to magnetization, and the R-rich phase is a non-magnetic phase that does not contribute to magnetization.
The alloy crystal structure containing a principal phase and an R-rich phase can be evaluated based on the spacing between adjacent R-rich phases (inter-R-rich phase spacing). In measurement of the inter-R-rich phase spacing, the cross section taken along the thickness direction (cross section in the thickness direction) of the produced alloy flake is examined, and the inter-R-rich phase spacing, which is a distance between an R-rich phase and an adjacent R-rich phase, is measured. Hereinafter, among R-rich phases, an R-rich phase enriched with Nd as a rare earth metal is also particularly referred to as an “Nd-rich phase”.
Rare earth alloy flakes produced by a rapid solidification process may be used as a material for rare earth sintered magnets and bonded magnets. If variations occur in the crystal structure of the rare earth alloy flakes that serve as the material and the distribution of the ferromagnetic principal phase and the non-magnetic R-rich phase is non-uniform, the resulting rare earth magnet will have decreased characteristics or suffer variations in product quality. Because of this, in the production of rare earth alloy flakes, there is a need for inhibiting the variations in the crystal structure of the resulting alloy flakes.
However, a chill roll that is used in casting of ribbons experiences a change in surface texture due to wear that is caused by repeated use in several casting operations. If the surface texture of a chill roll changes, the inter-R-rich phase spacing of resulting alloy flakes will vary. Thus, even if the alloy flakes are produced under the same casting conditions, the crystal structure of the alloy flakes will vary among individual casting operations, and this poses a problem.
With regard to ribbon casting by a rapid solidification process using a chill roll, there are various conventional proposals as disclosed in Patent Literatures 1 to 4, for example. Patent Literature 1 discloses a chill roll having a roll outer peripheral surface formed of a wear resistant metallic layer, the roll outer peripheral surface having a surface roughness Ra 2 of 0.1 to 10 μm in a widthwise central area thereof and a surface roughness Ra 1 of 2 to 20 μm in both side areas, wherein Ra 1 is greater than Ra 2. According to Patent Literature 1, this configuration inhibits variations in the crystal structure of the alloy between portions solidified on the central area of the chill roll and portions solidified on the side areas, thereby allowing production of alloy flakes having a fine and uniform crystal structure.
Patent Literature 2 discloses a chill roll having a roll outer peripheral surface configured such that the value of Sm/Ra defined by the mean spacing of profile irregularities Sm (mm) and the arithmetic mean roughness Ra (μm) is in the range of 0.03 to 0.12 (mm/μm), and that the mean spacing of profile irregularities Sm is in the range of 0.1 to 0.6 mm According to Patent Literature 2, this configuration homogenizes the crystal structure of the resulting rare earth alloy.
Patent Literature 3 relates to a method for repairing a chill roll that has been worn due to repeated use in several casting operations. The method of repairing a chill roll disclosed in Patent Literature 3 includes repairing a chill roll having a body provided with a thermally conductive layer on the outer periphery thereof and a metallic layer formed on the outer periphery of the thermally conductive layer, the method being performed by the following procedure:
(1) Removing a given amount of thickness from the outer peripheral surface of the chill roll;
(2) Conditioning the outer peripheral surface of the chill roll after removal of the given amount of thickness such that the center line average roughness is in the range of 1 to 50 μm; and
(3) Forming a metallic layer having a thickness determined by the thermal conductivity of the metallic layer to be formed, the thermal conductivity of the metallic layer on the outer peripheral surface from which a given amount of thickness was removed, and the center line average roughness of the outer peripheral surface from which a given amount of thickness was removed.
According to Patent Literature 3, by repairing the chill roll in accordance with the above steps (1) to (3), it is possible to return the chill roll to the state in which it provides cooling performance substantially comparable to that of a newly manufactured chill roll, and therefore to retain the quality of resulting alloy flakes stably and for a long period of time.
Patent Literature 4 discloses a chill roll having an outer peripheral surface configured to have a surface roughness represented by a ten point height of irregularities (Rz) of 5 to 100 μm. According to Patent Literature 4, by using a chill roll having irregularities on its outer peripheral surface, it is possible to prevent the ribbon surface that is brought into contact with the chill roll from being excessively quenched and to inhibit the production of fine R-rich phases near the ribbon surface that is brought into contact with the chill roll. It is stated that this configuration provides a homogeneous dispersion of R-rich phases in the side of the ribbon surface that is brought into contact with the chill roll and in the opposite side of the ribbon surface.